Grinding treatment of clay

ABSTRACT

A forth flotation process for the removal of mineral impurities from a crude clay wherein the flotation reagents are removed from the purified clay particles by agitation of a slurry of the clay with a particulate grinding material consisting of particles of a size in the range from one-half inch to 100 mesh B. S. sieve. The weight ratio of particulate grinding material to clay is in the range 2:1 to 5:1, and the agitation is continued for a time sufficient to dissipate in the slurry at least 100 horsepower hours of energy per long ton of clay.

[73] Assignee United States Patent 72] Inventor Norman Owen Clark Cornwall, England [2]] Appl. N0. 12,139

[221 Filed [45] Patented Feb. 17, 1970 Aug. 17, 1971 English Clays Lovering Pochin & Company Continuation-impart of application Ser. No. 732,386, May 27, 1968, now abandoned.

[54] GRINDING TREATMENT OF CLAY 6 Claims, 2 Drawing Figs.

[51] lnt. Cl ..B02c 17/04, B02c 21/00, 8020 23/06 50 Field of Search 241/4, 15, 16, 20,21, 22, 30

[ 56] References Cited UNITED STATES PATENTS 2,920,832 1/1960 Duke 241/21 X 3,464,634 9/1969 Brociner. 241/4 Primary Examiner-Donald 0. Kelly Attorney-Stevens, Davis, Miller and Mosher ABSTRACT: A forth flotation process for the removal of mineral impurities from a crude clay wherein the flotation reagents are removed from the purified clay particles by agitation of a slurry of the clay with a particulate grinding material consisting of particles of a size in the range from one-half inch to 100 mesh B. S, sieve. The weight ratio of particulate grinding material to clay is in the range 2:1 to 5:1, and the agitation is continued for a time sufficient to dissipate in the slurry at least 100 horsepower hours of energy per long ton of clay.

' PATENTE D A us l H971 SHEET 1 [1F 2 NOE GRINDING TREATMENT OF CLAY This application is a continuation-in-part of Ser. No. 732,386, filed May 27, 1968 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the treatment of clay and, more particularly but not exclusively, is concerned with the treatment of kaolinitic clays.

Owing to an increasing demand for high quality white clays for use in the paper coating industry, it is desirable to be able to. use deposits of inferior quality kaolinitic clays and to improve their brightness by an economic process. Various processes for improving the brightness of inferior quality kaolinitic clays have been tried, for example flotation or calcination, with varying degrees of success. Some processes are effective on some kaolinitic clays and not on others; and some processes produce kaolinitic clays which although having improved brightness have other unsuitable characteristics, for example calcined kaolinitic clays have a high brightness but are abrasive.

If a froth flotation process, such as is described in U.S. Pat. Specification No. 2,990,958 or No. 3,450,257 (U.K. Specification No. 1,103,585,) is used to treat a clay feedstock containing mineral impurities, it ispossible to obtain useful increases in brightness on many types of clay feedstocks. However, with the products of a froth flotation process, and particularly with those clay feedstocks where large quantities of reagents are required, the full potential of the process is not reached because the frothflotation reagents, e.g. oleic acid, are adsorbed by the clay and (a) give rise to frothing difficulties in the subsequent processing of the clay, (b) reduce the gain in clay brightness which would otherwise be obtained, and (c) reduce the gloss of a paper coated with a paper coating composition containing the clay. The reduction in the gain in the brightness of a clay, and the. reduction in the gloss of a paper coated with a paper coating composition containing a clay, on which there is adsorbed one or more of the reagents used in a froth flotation process can be demonstrated simply by comparing the properties of a clay before and after the adsorption ofa froth flotation reagent thereupon. For example, a

sample of clay comprising 65 percent by weight of particles smaller than 2 microns equivalent spherical diameter had a brightness of 82 units and when incorporated in a paper coating composition and coated onto paper under standard conditions gave a sheet gloss of 40 T.A.P.P.I. gloss units, whilst another sample of the same clay to which had been added 6 lbs. of oleic acid per ton of clay had a brightness of 80.5 units and when incorporated in a paper coating composition and coated onto paper under the same standard conditions gave a sheet gloss of 34 T.A.P.P.I. gloss units. It is therefore an object of the present invention to provide an improved froth flotation process for the treatment of a clay in which either the product of the process does not have deposited thereon significant amounts of one or more of the reagents used in the process or the effects of such reagents are nullified.

SUMMARY OF THE INVENTION In accordance with the present invention there is provided a process for increasing the brightness of a clay feedstock, which process comprises the steps of:

a. subjecting the clay feedstock, after conditioning and in the form of a reagentized aqueous slurry, to a froth flotation process so as to separate mineral impurities from said clay feedstock and to form a suspension of purified clay in a reagentized aqueous liquor.

b. adjusting the concentration of the suspension of purified clay in the reagentized aqueous liquor, if necessary, to a solids content of at least 20 percent by weight,

c. mixing the suspension of purified clay with a particulate grinding material in a volume ratio of particulate grinding material to suspension in the range 0.5:1 to 1.511, the particulate grinding material consisting of particles of a size in the range from inch to 100 mesh B.S. sieve,

d. agitating the mixture resulting from step (c) for a time suflicient to dissipate therein at least horsepower hours of energy per long ton of clay, and

e. separating the particulate grinding material and the clay.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the invention is particularly useful for treating feedstocks of kaolinitic clays intended for use in the paper coating industry. Such clays will normally have an initial particle size distribution such that at least 5 percent, but generally not more than 60 percent, by weight thereof consists of particles larger than 10 microns equivalent spherical diameter (e.s.d.).

The froth flotation process of step (a) can be carried out in a conventional manner, but advantageously is carried out according to one of procedures A and B described below:

PROCEDURE A the clay feedstock is formed into an aqueous slurry having a solids content of at least 30 percent by weight, preferably from 35 to 50 precent by weight, and then conditioned at said solids content by agitating the sulrry for a time sufficient to dissipate therein at least 20 horsepower hours of energy per long ton of clay. During or before the conditioning of the clay there is mixed with the aqueous clay slurry, a deflocculant, an alkali to raise the pH of the aqueous clay slurry to an alkaline value, a collector for the mineral impurities, and an activator. After conditioning of the clay, there is added to the clay slurry a frothing agent. The reagentized aqueous clay slurry thus formed is then introduced, preferably after dilution to a solids content in the range 15 percent to 30 percent by weight, into a froth flotation cell having a central, internal, submerged impeller wherein it is subjected to a froth flotation process. The purified clay is collected as the machine discharge product.

The deflocculant used in this procedure can be, for example, sodium silicate which is advantageously used in an amount varying from 1.0 to 12.0 lbs. per ton of clay.

The alkali used to raise the pH of the aqueous clay slurry to an alkaline value can be ammonium hydroxide although other similar materials may alternatively be used. In the case of the ammonium hydroxide there will generally be used from 1.0 to 12.0 lbs. per long ton of clay.

The collector for the mineral impurities can be, for'example a fatty acid containing from 8 to 20 carbon atoms. Preferred collectors are oleic acid and lauric acid, and in the case of oleic acid there will generally be used from 1.5 to 7.5 lbs. of oleic acid per long ton of clay.

The frothing agent can be, for example, pine oil or a methyl or ethyl ether or polypropylene glycol.

The activator can be, for example, a water-soluble salt of a heavy metal or an alkaline earth metal including magnesium, for example lead acetate or calcium chloride.

PROCEDURE B The clay feedstock is formed into an aqueous slurry having a solids content of at least 20 percent by weight, preferably from 35 to 50 percent by weight, and there is then added to the aqueous clay slurry a mineral acid to reduce the pH to 4.0 or below. A cationic collector is added to the aqueous clay slurry together with a frothing agent. The reagentized aqueous clay slurry thus formed is then conditioned at the solids content of at least 20 percent by weight, by agitating the slurry for a time, preferably but not essentially sufficient to dissipate therein at least 20 horsepower hours of energy per long ton of clay. The reagentized and conditioned aqueous clay slurry is then subjected to a froth flotation process, preferably after dilution where necessary to a solids content in the range 15 to 30 percent by weight, in a froth flotation cell having a central internal, submerged impeller. The froth containing the purifled clay is collected and sprayed with water under pressure to destroy the froth and leave the purified clay in'a flocculated state.

The cationic collector employed is advantageously an aliphatic amine having from 8 to 20 carbon atoms or an acetate or hydrochloride of such an amine, and is advantageously used in an amount in the range of from 0.2 to 2.0 lbs. of collector per long ton of clay.

The frothing agent can be, for example, pine oil or a methyl or ethyl ether of polypropylene glycol which is employed in an amount which is approximately the same as the amount of collector employed.

In both Procedures A and B above the froth flotation cell can be, for example, the Denver Sub-A machine shown on page 12-64 of the Handbook of Mineral Dressing-Ores and Industrial Minerals" by Taggart.

At the end of the froth flotation process of step (a) the purifled clay is generally recovered in the form of a suspension, in a reagentized aqueous liquor, having a solids content of less than 30 percent by weight. If the solids content is less than 20 percent by weight, the suspension must be concentrated to at least 20 percent. Preferably the suspension is concentrated to a solids content of at least 65 percent by weight to remove a major portion of the reagentized liquor. This can be achieved by first decanting the suspension, in the case of an anionic flotation process after treatment with a mineral acid to flocculate the clay, to obtain a thickened suspension and then filterpressing the thickened suspension to obtain a filter cake having a solids content of at least 65 percent by weight.

The clay suspension is reslurried, if necessary, optionally with a nonmucilaginous dispersing agent, to a solids content in excess of 20 percent, generally from 25 percent to 50 percent by weight and preferably to a solids content of at least 35 percent by weight. There can then be added to the slurry a nonmucilaginous dispersing agent, preferably a water-soluble salt having a macromolecular anion with an average molecular weight in the range 100 to 10,000, for example a polyphosphate, e.g. that sold under the Trade Mark CAL- GON, tetrasodium pyrophosphate, a sodium salt of a polysilicic acid or a sodium, potassium or ammonium salt of a polyacrylic acid. When a nonmucilaginous dispersing agent is not used the solids content of the slurry preferably does not exceed 40 percent by weight. When a nonmucilaginous dispersing agent is used, the solids content of the slurry may be as high as 65 percent or more by weight. Generally, the amount of nonmucilaginous dispersing agent used will normally lie in the range of from 0.2 to 0.5 percent by weight, based on the weight of clay.

The aqueous clay slurry is then mixed with a particulate grinding material in a volume ratio of particulate grinding material to clay suspension in the range of from 0.521 to 1.5:], the particulate grinding material consisting of particles ranging in size from A inch to 100 mesh B.S. sieve. Preferably, the grinding material consists of particles ranging in size from a No. 10 mesh B.S. sieve to a No. 30 mesh B.S. sieve. The particulate grinding material can, with advantage, be for example sand or a calcined china clay. Advantageously, the weight ratio of the particulate grinding material to the clay is in the range 2:1 to :1 most preferably about 4:1.

The mixture of aqueous clay slurry and grinding material is then agitated or a time sufficient to dissipate therein at least 100 horsepower hours of energy, per long ton ofclay.

When the grinding process has been completed the clay is separated from the particulate grinding material, for example by sieving.

In order to obtain the significant improvements in brightness which are achieved by the present invention, it is essential that the products of the froth flotation process are ground in the manner described herein, since it has been found that other grinding processes do not achieve any significant improvement in the brightness of the clay product over and above that which could be obtained by treating the froth flotation product by a chemical bleaching process.

The invention is further illustrated by the following Examples in which reference is made to FIGS. 1 and 2 of the accompanying drawings which show schematically two installations for carrying out a process in accordance with the invention.

EXAMPLES 1 In this Example the installation shown in FIG. 1 was employed.

A first sample of an impure china clay comprising 60 percent by weight of particles smaller than 2 microns equivalent spherical diameter and having a percentage reflectance to violet light of 458 millimicrons wavelength of 81.0 units was subjected to the froth flotation process described below. The impure clay I was mixed wit water 2 in a vessel 3 to give an aqueous slurry containing 38 percent by weight of solids. The aqueous slurry was fed via a conduit 4 to a vessel 5 wherein it was admixed with oleic acid 6, which was added in an amount of 4.5 lbs. of oleic acid per long ton of clay, ammonium hydroxide 7, which was added in an amount of 3.5 lbs of ammonium hydroxide per long ton of clay, lead acetate 8, which was added in amount of 1.4 lbs. of lead acetate per long ton of clay, and sodium silicate 9, which was added in an amount of 6.0 lbs. of sodium silicate per long ton of clay. The reagentized aqueous clay slurry was then conditioned by agitating for 12 minutes under conditions which resulted in the dissipation therein of 23 horsepower hours of energy per long ton of clay. At the end of the conditioning period water was added at l 1 to dilute the reagentized slurry to a solids content of 18 percent by weight and the slurry then introduced into a froth flotation cell 13 having a width of 36 inches and provided with a central, internal submerged impeller of diameter 18 inches which was arranged to rotate at a peripheral speed of 1,050 feet per minute. 1.5 lbs. of pine oil per long ton of clay was then added as a frothing agent 12. The resulting mixture was subjected to a froth flotation process for a period of 60 minutes and the machine discharge product containing purified china clay was then bleached with zinc hydrosulphite and refined.

The foregoing procedure was repeated on a second sample of the same impure china clay except that, at the end of the froth flotation process and before bleaching, the machine discharge product was fed by a conduit 14 to a thicker 17, a mineral acid being added at 16 in order to flocculate the clay. The flocs were allowed to settle and the supernatant water decanted ofi'. The thickened clay was then fed via a pipe 18 to a filter press 19 wherein it was compacted to form a filter cake having a solids content of about 70 percent by weight. The filter cake was reslurried in a vessel 20 with water 21 and 0.3 percent by weight of tetrasodium pyrophosphate 22 to give a slurry containing 45 percent by weight of solids. This slurry was then fed to a grinding apparatus 23 comprising a rubber lined cylinder wherein it was mixed with granules of a calcined china clay, having a size intermediate a No. 10 mesh B.S. sieve and a No. 30 mesh .B.S. sieve, in the volume ratio of 0.95 volumes of calcined china clay granules to one volume of the suspension of china clay corresponding to a weight ratio of four parts by weight of calcined china clay granules to one part by weight of china clay from the froth flotation process. The cylinder 23 was sealed, placed on rollers and rotated at 61 r.p.m. for a time sufficient to dissipate in the slurry 250 horsepower hours of energy per long ton of clay. At the end of the grinding operation the china clay from the froth flotation process and the coarser calcined china clay used as grinding material were separated by sieving.

Finally, a third sample of the same impure china clay was treated in a similar way except that, after carrying out the froth flotation process and before bleaching, the clay was subjected to a ball milling process. In this process, the machine discharge product was concentrated and formed into a slurry having a solids content of 45 percent by weight using the same procedure as described above. To the slurry there was added 0.3 percent by weight of tetrasodium pyrophosphate and the resulting dispersion introduced into a rubber-lined cylinder containing flint pebbles of diameter ranging between 1% inch to as inch. The volume ratio of flint pebbles to china clay suspension was 0.95:1 the weight of the charge of flint pebbles being four times the weight of china clay in the suspension. The rubber-lined cylinder was then sealed, placed on rollers and rotated at 61 rpm. for a time sufficient to dissipate in the slurry 250 horsepower hours of energy per long ton of clay. Thereafter the flint pebbles and china clay were separated.

The percentage reflectance to violet light of 458 millimicrons wavelength of the three clay samples, the gloss of a paper coated with a paper coating composition containing the clay samples, were measured and are recorded in Tables 1A and 1B below.

TABLE 1A Brightness Increase in Percentreflectance brightness age of to violet particles light of 458 smaller m wavelength than 2 First Sample: a. Feed to process 8L0 60 b. Froth flotation product 87.0 6.0 c. Bleached product 87.5 6.5 d. Refined product" 890 8.0 90

Second Sample: a. Feed to process 8 t .0 60 b. Froth flotation product 87.0 6 c. Ground product 89.6 8.6 d. Bleached and refined product"(88) 9L0 10.0 90

Third Sample: a. Feed to Process 8L0 60 b. Froth flotation product 87.0 6 c. Ball milled product 87.8 6.8 d. Bleached and refined product" 89.3 8.3 90

(l) The refined product was obtained by panicle size separation using a centrifuge. In the case of the first sample there was a 48% recovery, in the case of the second sample there was a 95% recovery. and in the case of the third sample there was a 79% recovery.

TABLE I8 Gloss of paper coated with paper coating composition employing clay samples First Sample 59 TAPPl Gloss Units Second Sample 68 TAPPl Gloss Units Third Sample 62 TAPPI Gloss Units Sta n clar cl sheets of paper were coated to a weight of 10 g.s.m. with a paper coating composition comprising the following ingredients:

Clay Sample 100 parts by weight Starch l pans by weight Styrene-butadiene latex l0 parts by weight Water 40 parts by weight (ALCON (Trade Mark) 0.3% by weight based on weight ofclay.

EXAMPLE 2 lnthis Example the installation shown in FIG. 2 was employed.

A coarse residue china clay 1 containing, as impurities, mica, quartz and feldspar, was admixed with water 2 in a vessel 3 to give an aqueous slurry containing 40 percent by weight of solids. Sufficient dilute hydrochloric acid 26 was added to reduce the pH of the slurry to 3.0. The aqueous slurry was then transferred to a vessel 5 and admixed therein with ootadecylamine acetate 27, which was added at the rate of 0.8 lbs. per long ton of clay, and pine oil 12, which was added at the rate of 0.8 lbs. per long ton of clay. The reagentized aqueous slurry was then conditioned by agitating for a period of 12 minutes under conditions which resulted in there being dissipated in the slurry 24 horsepower hours of energy per long ton of clay. At the end of the conditioning period water 11 was added to dilute the slurry to 25 percent by weight of solids, the slurry then being fed to a froth flotation call 13 similar to that used in Example 1. The cell was run for 60 minutes and the froth 14, which comprises kaolinitic clay and mica substantially free from abrasive material, was continuously removed, further additions of water being made at intervals to maintain a substantially constant level of pulp in the cell. The collected froth was sprayed with high pressure water jets 24 in order to suppress the froth and release the product in the form of a flocculated suspension.

The product of the process was divided into three samples, the first of which received no further treatment and the second and third of which were subjected to a grinding process, as described below, for lengths of time sufficient to dissipate therein and horsepower hours of energy per long ton of clay, respectively. More particularly, the second and third samples were treated in the following way:

The flocculated suspension was first concentrated by transferring it via conduit 25 to a thickener 17 wherein the flocs were .allowed to settle under gravity, the supernatant water being decanted off. The thickened suspension was then transferred by means of a conduit 18 to a filter press 19 wherein it was compacted to form a filter cake having a solids content of about 70 percent by weight. The filter cake produced in this way was then reslurried with water containing 0.3 percent by weight of tetrasodium pyrophosphate to give a slurry containing 40 percent by weight of solids. 1,880 ml. of this suspension were then introduced into a rubber-lined cylinder 23 of 6.6 litres capacity together with 4,000 g. of granules of the calcined china clay used in Example 1 to give a volume ratio of particulate grinding material to clay/water suspension of 0.81:1 the weight ratio of particulate grinding material to clay mineral being 4:] The cylinder 23 was then sealed, placed on rollers, and rotated for a time sufficient to dissipate therein a specified amount of energy. Samples of l) the original coarse china clay, (2) the unground froth flotation product, (3) the froth flotation product ground at 80 horsepower hours of energy per ton of clay, and (4) the froth flotation product ground at 160 horsepower hours of energy per long ton of clay, were then tested (a) for the weight per cent of particles having an equivalent spherical diameter smaller smaller than 2 microns, (b) for the weight per cent of particles having an equivalent spherical diameter greater than 10 microns, (c) for brightness in terms of the percentage reflectance to violet light of 458 millimicrons wavelength before and after bleaching with zinc dithionite, and (d) for abrasiveness. The results obtained are set out in table 2A below:

I The abrasiveness was measured by the Valley Abrasion test, and the higher figures represent a higher degree ol'abrasiveness.

A mineralogical analysis was also made by X-ray diffraction, the results obtained being recorded in table 28.

7 8 TABLE 28 A further quantity of the same residue china clay was treated in an identical manner except that, after the froth Sample No. l 2 s 4 product had been thickened by gravity in a tank and then filter pressed, the filter cake was mixed with a smaller quantity of Mineral y weight 5 water and no dispersing agent to give a suspension containing 45 percent by weight of solids. 340 imperial gallons of this Kaolinitc 72 77 77 77 suspension were mixed with 43 cwt. of the same silica sand but Mica 2s 23 23 23 it was discovered that the resultant mixture was too viscous to 2 0 0 be agitated in the same grinding mill. Feldspar l 0 0 0 1 0 I EXAMPLE 4 EXAMPLE 3 A residue china clay containing, as impurities, mica, quartz A residue china clay containing, as impurities, mica, qu r z and feldspar and having a particle size distribution such that 9 and feldspar and having a particle size distribution such that 6 15 percent by weight consisted of particles smaller than 2 percent by weight consisted of particles smaller than 2 microns equivalent spherical diameter and 39 percent by microns equivalent spherical diameter and 45 percen by weight of particles larger than 10 microns equivalent spherical weight particles larger than 10 microns equivalent spherical di m t wa f d t a f th fl i plant i h f m f an diameter was fed to a plane similar to that shown in FIG. 2 in aqueous suspension comprising 25 percent by weight of the the form of an aqueous suspension comprising 25 percen by dry china clay. To this suspension there were added 0.3 lb. of weight of dry china clay. To this suspension there were added ARMAC HT per long ton of dry china clay and sufficient sula cationic flotation collector, ARMAC HT manufactured by furic acid to lower the pH to 2.6. The mixture was conditioned Armour & Co., at a dose of 0.3 lb. of the collector p r l ng in a stirred tank. There was then added 0.4 lb of a frother comof dry clay, and sufficient sulfuric acid to lower the pH to prisingamethyl either of polypropylene glycol per long ton of about 3; and the mixture was conditioned in a stirred tank. dry china clay and the mixture was subjected to continuous Pi il wa then added as a frother at a dose of 0.3 lb of pine froth flotation in four stages, the feed to each successive stage oil per long ton of dry clay and the mixture wa Subjected to being the machine discharge product from the previous stage. continuous froth flotation in four stages, the feed to each suc- Th froth from the flotation cells in all four stages was colcessive stage being the machine discharge product from the l d d Sprayed i h fin j f water d h machine previous stage. The froth from th fl i Cells in four discharge product from the fourth stage cell was discarded as stages was collected and sprayed with fine jets of water and waste, the machine discharge product from the cell or cells in the The froth product was thickened by gravity in a tank to give fourth stage was discarded as waste. a suspension containing 32 percent by weight of solids. This The froth product was thickened by gravity in a n and suspension was then diluted with clean water to 21 percent by filter Pressed to give a cake containing 69 Percent y 35 weight of solids and the diluted suspension was mixed with silweight y china clay. The filter cake was mixed with ica sand consisting of particles having sizes in the ranges 16 to Sufi-mien! Water to g a Suspension containing 27 Percent by 30 mesh 85. sieve in the proportion of 16 cwt. of sand to 135 weight of Solids and dispersing agent was added- 340 imperial gallons of suspension. The weight ratio of dry sand to ions of this suspension and 43 cwt. of silica sand consisting of dry china clay was th f 5 and the volume 'ratio f p having sizes in the range 16 to 30 mesh sieve sand to china clay/water suspension was 0.50:1. The mixture were loaded into a sand grinding mill and agitated for a time was agitated in a sand grinding mill for a time Sumciem to sufiicient to dissipate in the suspension 150 horsepower hours sipate to h ion 165 horsepower ho of energy l of energy P lohg of dry clay The weight ratio of dry Sand long ton of dry china clay. The suspension of ground clay was to dry water suspension was 0.54:1. The suspension of ground 45 then Separated f the Sand by sieving Clay was separated from the send by sieving- Samples of the feed to the flotation plant, the front product samPles of the feed to the flotation Plant the froth product and the sand-ground froth product were tested for percentage and the sand ground froth Ptoduet were tested for Percent reflectance to light of 458 and 574 millimicrons wavelenghts reflectance hght of 458 and 574 millimicrons wavelengths and for particle size distribution. In order to determine the and for particle size distribution. In order to determine the Susceptibility of the samples to chemical bleaching, each was Susceptibility of the Samples to chemical bleaching each was treated in aqueous solution at pH 2.8 with sodium dithionite. treated in aqueous Solution at P with sodium dithionite? The dose of sodium dithionite in each case corresponded to 4 the dose of Sodium dithionite each case corresponding to 4 lbs. of dithionite per long ton of dry china clay. The bleached lb. of the dithionite per long ton of dry clay. The bleached clay clay was then filt d washed and dried at 30 and the was filtered, washed and dried at and the Percent centage reflectance to light of 458 and 574 millimicrons reflectance to light of 453 and 574 millimicrons wavelength wavelength was determined. The results are shown in table 4 was determined. The results obtained are shown in table 3 below below.

TABLE 4 TABLE 3 Feed Froth Sand ground Feed Froth Sand ground product product by wt. smaller I: by weight smaller than 2 microns e.s.d. 6 8 30 than 2 microns e.s.d. 9 ll 3] X: by wt. larger 21 by weight larger than 10 microns e.s.d. 45 36 8 than 10 microns e.s.d. 39 33 7 reflectance to light 11 reflectance to light of 458 mp (unbleached) 77.0 78.4 82.7 of 458 my. (unbleached) 76.9 78.8 82.! reflectance to light k reflectance to light of 574 my. (unbleached) 82.5 ms 87.5 M574 m (unbleached) 82.2 84.9 87.9 X- of reflectance In light I; of reflectance hush m, (bleached) 77.7 no.2 34.1 to light 01'458 m (blanched) 77.5 79.2 83.3 t reflcclunce to light b reflectance to light [574 Ill .4 (bleached) 32.7 84.2 87.9 M574 my (bleached) 82.3 84.6 87.9

EXAMPLE A residue china clay containing, as impurities mica, quartz and feldspar and having a particle size distribution such that 5 percent by weight consisted of particles smaller than 2 microns equivalent spherical diameter and 55 percent by weight of particles larger than microns equivalent spherical diameter was fed to a flotation plant in the form of an aqueous suspension comprising 25 percent by weight of the dry china clay. To this suspension there were added 0.3 lb. of ARMAE HT per long ton of dry china clay and sufficient sulfuric acid to lower the pH to 2.6. The mixture was conditioned in a stirred tank. There was then added 0.4 lb. of the same frother as in Example 3 per long ton of dry china clay and the mixture was subjected to continuous froth flotation in four stages, the feed to each successive stage being the machine discharge product from the previous stage. The froth from the flotation cells in all four stages was collected and sprayed with fine jets of water and the machine discharge product from the fourth stage cell was discarded as waste.

The froth product was thickened by gravity in a tank and filter pressed to give a cake containing 69 percent by weight of dry china clay. The filter cake was then divided into portions the first of which was diluted with water containing 0.45 percent by weight of sodium silicate dispersant based on the weight of dry china clay to give a suspension comprising 51 percent by weight of solids. The second was diluted with water containing the same quantity of sodium silicate to give a suspension comprising 60 percent by weight of solids and the third was mixed with the same quantity of water so that the final solids content of the suspension was still nearly 69 percent by weight. The first suspension was mixed with silica sand consisting of particles having sizes in the range 16 to 30 mesh B.S. sieve in the proportion l cwt. of sand to 7 imperial gallons of suspension. The weight ratio of dry sand to dry china clay was therefore 21:1 and the volume ratio of the sand to china clay/water suspension was 0.61:1. The mixture was agitated in a sand grinding mill for a time sufficient to dissipate in the suspension 105 horsepower hours of energy per long ton of dry china clay.

The second suspension was mixed with. the same sand in the same proportions by volume but the weight ratio of dry sand to dry china clay was now 1.6: l. The mixture was agitated in a sand grinding mill for a time sufficient to dissipate in the suspension 135 horsepower hours of energy per long ton of dry china clay.

The third suspension was also mixed with the same sand in the same proportions but the mixture was found to be so viscous that it was impossible to agitate it in the sand grinding mill.

The first and second suspensions were separated from the sand by sieving and samples of the feed to the flotation plant, the froth product and the sand ground products 1 and 2 from the first and second suspension respectively were tested for percentage reflectance to light of 458 and 574 millimicrons wavelength and for particle size distribution. In order to determine the susceptibility of the samples to chemical bleaching each was treated in aqueous solution at pH 2.8 with sodium dithionite. The dose of sodium dith'ionite in each case corresponding to 4 lbs. of the dithionite per long ton of dry clay. The bleached clay was then filtered, washed and dried at 80 C. and the percentage reflectance to light of 458 and 574 millimicrons wavelength was determined. The results are shown in table 5 below:

Table 5 C0ntinued As previously noted, the clay product of the process of the present invention is of particular value for use in the paper coating industry.

I claim:

1. A process for increasing the brightness of a crude clay, which process comprises the steps of:

i. conditioning the crude clayand thereafter subjecting the crude clay in the form of a reagentized aqueous slurry to a froth flotation process so as to separate mineral impurities from said crude clay and to form a suspension of purified clay in a reagentized aqueous liquor,

ii. concentrating the suspension of purified clay in the reagentized aqueous liquor to a solids content of at least 65 percent by weight to remove therefrom a major proportion of the reagentized liquor,

iii. forming the concentrated suspension of purified clay into an aqueous slurry having a solids content in the range of from 25 percent to 50 percent by weight and containing a nonmucilaginous dispersing agent,

iv. mixing the aqueous slurry of purified clay containing the nonmucliginous dispersing agent with a particulate grinding material in a weight ratio of particulate grinding material to clay in the range 2:1 to 5:1, the particulate grinding material consisting of particles ranging in size from inches to 100 mesh B.S. Sieve,

v. agitating the mixture resulting from step (iv) for a time sufficient to dissipate therein at least 100 horsepower hours of energy per long ton of clay, and

vi. separating the particulate grinding material and clay.

2. A process according to claim 1, wherein the particulate grinding material consists of particles ranging in size from a No. 10 meshllS. sieve to a No. 30 mesh B.S. sieve.

3. A process according to claim 2, wherein said mixture resulting from step (d) is agitated for a time sufficient to dissipate therein at least 150 horsepower hours of energy per ton of clay.

4. In a process for increasing the brightness of a clay feedstock wherein the clay feedstock is conditioned and then subjected in the form of a reagentized aqueous slurry to a froth flotation process so as to form a suspension of purified clay in a reagentized aqueous liquor, the improvement which comprises the steps of a. adjusting the concentration of the suspension of purified clay in a reagentized aqueous liquor to a solids content of at least .20 percent by weight,

b. mixing the aqueous suspension of purified clay with a particulate grinding material in a volume ratio of particulate grinding material to clay/water suspension in the range 0.5 to 1.5.: 1., the particulate grinding material consisting of particles of a size in the range from inches to 100 mesh B.S. sieve,

c. agitating the mixture resulting from step (b) for a time sufficient to dissipate therein at least 100 horsepower hours of energy per'ton of clay, and

d. separatingthe particulate grinding material and the elay.

5. A process according to claim 4, wherein the suspension of purified clay in a reagentized aqueous liquor is concen- 75 trated to a solids content of at least 65 percent .by weight to remove a major proportion of the reagentized aqueous liquor and is thereafter formed into an aqueous slurry having a solids content in the range from 20 to 65 percent by weight and containing a nonmucilaginous dispersing agent, prior to mixing in the particulate grinding material in step (b).

6. A process according to claim 4, wherein the suspension 

2. A process according to claim 1, wherein the particulate grinding material consists of particles ranging in size from a No. 10 mesh B.S. sieve to a No. 30 mesh B.S. sieve.
 3. A process according to claim 2, wherein said mixture resulting from step (d) is agitated for a time sufficient to dissipate therein at least 150 horsepower hours of energy per ton of clay.
 4. In a process for increasing the brightness of a clay feedstock wherein the clay feedstock is conditioned and then subjected in the form of a reagentized aqueous slurry to a froth flotation process so as to form a suspension of purified clay in a reagentized aqueous liquor, the improvement which comprises the steps of a. adjusting the concentration of the suspension of purified clay in a reagentized aqueous liquor to a solids content of at least 20 percent by weight, b. mixing the aqueous suspension of purified clay with a particulate grinding material in a volume ratio of particulate grinding material to clay/water suspension in the range 0.5 to 1.5:1, the particulate grinding material consisting of particles of a size in the range from 1/2 inches to 100 mesh B.S. sieve, c. agitating the mixture resulting from step (b) for a time sufficient to dissipate therein at least 100 horsepower hours of energy per ton of clay, and d. separating the particulate grinding material and the clay.
 5. A process according to claim 4, wherein the suspension of purified clay in a reagentized aqueous liquor is concentrated to a solids content of at least 65 percent by weight to remove a major proportion of the reagentized aqueous liquor and is thereafter formed into an aqueous slurry having a solids content in the range from 20 to 65 percent by weight and containing a nonmucilaginous dispersing agent, prior to mixing in the particulate grinding material in step (b).
 6. A process according to claim 4, wherein tHe suspension of purified clay in a reagentized aqueous liquor is concentrated to a solids content of at least 65 percent by weight to remove a major proportion of the reagentized aqueous liquor and is thereafter formed into an aqueous slurry having a solids content in the range from 20 to 40 percent by weight, prior to mixing with the particulate grinding material in step (b). 